This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled Electron Gun For Cathode Ray Tube earlier filed in the Korean Industrial Property Office on Nov. 19, 1999, and there duly assigned Ser. No. 99-51494 by that Office.
1. Field of the Invention
The present invention relates to a color cathode ray tube (CRT), and more particularly, to an electron gun for a color CRT having improved electrodes for forming an electronic lens.
2. Description of the Related Art
An electron gun used for a large-screen color CRT must satisfy a requirement of generating both low-current electron beams and high-current electron beams in a stable manner. A conventional CRT employs a self-converging deflection yoke having a pin-cushion type deflected magnetic field and a barrel-type deflected magnetic field, and an in-line electron gun. The deflected magnetic field of the deflection yoke over-focuses an electron beam vertically and under-focuses an electron beam horizontally, to cause focus separation. The deformed electron beam spot becomes asymmetrical when it is deflected to the periphery of a screen. Also, an in-line electron gun cannot attain focusing uniformity due to a change in the intensity of an electronic lens caused by a change in the focusing voltage.
In order to prevent a focus deterioration of an electron beam landing on a phosphor layer, an apparatus in which the cross section of an electron beam emitted from an electron gun is made vertically elongated to compensate for distortion due to a non-uniform magnetic field of a deflection yoke, has been proposed.
FIG. 1 shows an example of electrodes of an electron gun for deforming the cross section of an electron beam as disclosed in U.S. Pat. No. 4,772,827 to Kuniharu Osakabe and entitled Cathode Ray Tube and incorporated by reference herein.
As shown in the drawing, circular electron beam apertures 11 are formed on a first electrode 10, and four vertical blades 12 are formed at either side of the electron beam apertures 11. Circular electron beam aperture 21 are formed on a second electrode 20 which is disposed opposite to the first electrode 10, and two single horizontal blades 22 are respectively installed above and below the circular electron beam apertures 21. A predetermined focusing voltage VF is applied to the first electrode 10, and a dynamic focusing voltage VFd synchronous to a deflection signal produced when an electron beam is deflected toward the periphery of the screen is applied to the second electrode 20.
In the first and second electrodes 10 and 20 of an electron gun for forming the above-described conventional quadrupole (four pole) lens, the quadrupole lens formed by application of the focusing voltage and the dynamic focusing voltage synchronous to the deflection signal is affected by the blades 12 and 22 so that distortion of the quadrupole lens becomes severe, which causes a vertical haze when astigmatism is relatively elongated horizontally, resulting in deterioration in focusing characteristics over the entire screen. In particular, when a dynamic focusing voltage of a high waveform is applied, a haze due to vertical over-focusing of an electron beam is generated, in spite of improvement in horizontal focusing characteristics.
In order to form a quadrupole lens, a relatively high waveform voltage must be applied to the electrodes 10 and 20, which causes an increase in the cost required for circuit construction. Also, since the blades 12 and 22 are installed to be opposite to each other, it is quite difficult to assemble an electron gun employing such electrodes.
Another example of electrodes of an electron gun for a conventional color CRT is illustrated in U.S. Pat. No. 4,814,670 to Hiroshi Suzuki et al. and entitled Cathode Ray Tube Apparatus Having Focusing Grids With Horizontally And Vertically Oblong Through Holes, in which three vertically elongated electron beam apertures are formed on the exit side of a first focus electrode for forming a quadrupole lens, and a horizontally elongated electron beam aperture through which all of three electron beams pass is formed on the entrance side of a second focus electrode installed to face the first focus electrode, for forming the quadrupole lens. A static focusing voltage is applied to the first focus electrode and a dynamic focusing voltage synchronous to a deflection signal is applied to the second focus electrode.
In the above-described electron gun, since a single horizontally elongated electron beam aperture is formed on the second focus electrode, the intensities, that is, magnifications, between the center of an electronic lens formed by the first and second focus electrodes and either side thereof are different. Thus, the sizes of electron beam spots landing on left and right sides of a screen are different, causing a moire.
Additional references describing electron guns using quadrupole lenses for preventing astigmatism and incorporated by reference herein are: U.S. Pat. No. 5,656,884 to Soo Keun Lee entitled Electron Gun Of A Color Picture Tube For Preventing Astigmation; U.S. Pat. No. 5,394,053 to Neung-yong Yun entitled Electron Gun For A Color Cathode Ray Tube; U.S. Pat. No. 5,300,855 to Yong-geol Kweon entitled Electron Gun For A Color Cathode Ray Tube; and U.S. Pat. No. 5,170,101 to Richard M. Gorski et al. entitled Constant Horizontal Dimension Symmetrical Beam In-Line Electron Gun.
To solve the above problems, it is an object of the present invention to provide an electron gun for a color CRT, which can reduce a dynamic focusing voltage and can improve the resolution of an image by reducing the astigmatism of an electronic lens.
To accomplish the above object, there is provided an electron gun for a color cathode ray tube comprising a cathode for emitting electron beams, a control electrode and a screen electrode, sequentially installed from the cathode, constituting a triode, a first focusing electrode having vertically elongated electron beam apertures, disposed next to the screen electrode to form a quadrupole lens, a second focusing electrode having circular electron beam apertures, and a final accelerating electrode disposed next to the second focusing electrode to form a main lens, wherein a static voltage is applied to the first focusing electrode and a dynamic focusing voltage synchronous to a deflection signal is applied to the second focusing electrode.
In the present invention, the voltage applied to the first focusing electrode is 200 to 1500 V higher than that applied to the second focusing electrode.